High-pressure fuel pumps which highly pressurize a fuel supplied to a combustion chamber of an internal combustion engine are usually formed as piston pumps. In such pumps, a piston compresses a fuel in a pressure chamber by means of a translational movement and thus highly pressurizes it. For example, typical high-pressure fuel pumps in petrol internal combustion engines pressurize the fuel to a pressure between 200 bar and 250 bar, while high-pressure fuel pumps in diesel internal combustion engines pressurize the fuel to a pressure between 2000 bar and 2500 bar.
In order to drive the piston in its translational movement, a camshaft is commonly used as a drive shaft which rotates about a drive shaft rotational axis so that a cam projecting beyond the drive shaft periodically pushes the piston away from the drive shaft rotational axis. The volume of the pressure chamber, in which the fuel is located, is reduced and the fuel is thus pressurized. In the case of a further rotation of the drive shaft, the piston then moves again in the direction of the drive shaft rotational axis, as a result of which the volume of the pressure chamber is increased again.
There are various approaches for transmission of the translational movement from the drive shaft to the piston. One example is an eccentric ring, what is known as a rider, often used in contact with the drive shaft such that the drive shaft rotates away together with the cam under the eccentric ring, while the eccentric ring is moved up and down in a translational direction without itself rotating. The eccentric ring has a flat surface which is in operative contact with the piston, and indeed usually via a sliding shoe which slides over the flat surface of the eccentric ring and thus transmits the movement of the eccentric ring to the piston.
Such an arrangement is shown in FIG. 3 and FIG. 4. FIG. 3 is a longitudinal sectional view through a high-pressure fuel pump 10 according to the prior art, while FIG. 4 shows a sectional view through high-pressure fuel pump 10 from FIG. 3 along line IV-IV.
High-pressure fuel pump 10 includes two pistons 12 which are opposite one another and delimit in each case a pressure chamber 14 on one side. Pressure chambers 14 are fed in each case via an intake 16, in which a first valve 18 is arranged, with a fuel which is highly pressurized by translational movement of piston 12. The highly pressurized fuel is then conducted in each case via an outlet 20, in which a second valve 22 is arranged, to a combustion chamber. Pistons 12 are driven by a drive shaft 24 which rotates about a drive shaft rotational axis 26. In order to be able to perform the piston stroke, the mechanical energy is transmitted in the form of rotational energy, i.e., a torque, into a translational movement of piston 12 by virtue of the fact that what is known as an eccentric drive 28 is interconnected between drive shaft 24 and piston 12.
Eccentric drive 28 has an eccentric ring 30 under which drive shaft 24 and a cam 34 arranged thereon rotate away. Eccentric ring 30 is periodically guided away in the direction of pressure chamber 14 and away from it without, however, itself rotating. A center point M of eccentric ring 30 is spaced apart by a distance A from rotational axis 26 or shaft center point W. With the rotation about W and eccentricity A=MW, a stroke of 2·MW is produced.
Eccentric drive 28 further has a sliding shoe 36 which slides on a flat eccentric ring surface 38 during the movement of eccentric ring 30 and passes on the translational movement of eccentric ring 30 to piston 12.
So that piston 12 and sliding shoe 36 are continuously in contact with eccentric ring surface 38, a spring 40 is provided which pretensions piston 12 and sliding shoe 36 onto eccentric ring surface 38.
Alternatively to eccentric drive 28 shown in FIG. 3 and FIG. 4, there are also arrangements in which, for example, a roller tappet rolls with a roller directly on cam 34 of drive shaft 24 and thus transmits the translational movement to piston 12. The advantage of these arrangements with a roller tappet is that in comparison with an eccentric drive 28 instead of sliding friction between eccentric ring 30 and sliding shoe 36 a significantly lower rolling friction is present between cam 34 and a roller of the roller tappet.
Roller tappets therefore have the advantage that lower friction is present between the individual elements of a drive region, but they are less robust than eccentric drives. Such eccentric drives can namely, for example, also be used in combination with connecting rods in an internal combustion engine which tends to be difficult when using roller tappets.